Fatigue Crack Growth and Life Predictions in Man-Ten Steel Subjected to Single and Intermittent Tensile Overloads

Abstract

Fatigue crack growth behavior was investigated in Man-Ten steel under conditions of constant load amplitude, single tensile overloads followed by constant load amplitude and intermittent single tensile overloads. Constant-amplitude ratios, Rl = Plmin / Plmax, were + 1/2, 0, −1/2, −1 and −2. The single overload test program excluded Rl = +1/2, and the intermittent overload program was for Rl = 0 only. A new simplified mathematical model was introduced which assumes that fatigue crack growth retardation is primarily a function of the overload ratio and the load ratio. The model predicted intermittent tensile overload life within 57 to 112 percent of the experimental life. The average prediction was 77 percent of experimental life. For Man-Ten steel, constant-amplitude crack growth rate versus positive ▵К fell within a reasonable scatter band for Rl = 1/2 to −1. However, substantial decrease in life occurred with Rl = −2. Negative values of Rl following a single tensile overload substantially reduced retardation life. Thus compressive stress, either large or small, cannot be neglected in spectrum loading fatigue crack growth life predictions.